1. Field of the Invention
The present invention relates to a cosmetic composition for darkening the skin and/or hair. The invention also relates to a method of topically applying to the skin and/or hair a skin/hair darkening composition according to the invention.
2. The Related Art
Skin tanning by UV exposure is a well known phenomenon. However, it is also well known from the literature that such exposure to UV radiation results in accelerated aging of skin and increased incidence of skin cancer. Accordingly, alternative modes of skin tanning have evolved. It is presently known in the art to use dihydroxy acetone (DHA) as a non-UV induced tanning aid. However, undesirably, the use of dihydroxy acetone for skin tanning purposes produces a rather unnatural looking sun tan. Further, the artificial tan produced by DHA does not protect against UV irradiation as would a natural tan.
Melanin is the black pigment of hair and skin and is synthesized from the amino acid tyrosine by melanosomes. Melanosomes are organelles found in melanocytes, a cell type present at dermis-epidermis junction. Tyrosine is acted upon by an enzyme, tyrosinase, which is the key step in melanogenesis.
In the melanosomes the melanin is synthesized from monomers and is transferred to the neighbouring cells called keratinocytes. The keratinocytes divide and differentiate and thus transport the melanosome to the surface of the skin. The intensity of the skin colour is directly related to the number, the size, melanin content, the rate of formation and migration/transfer of melanosomes to keratinocytes.
Several specific sequences of polyaminoacids and peptide residues are known to inhibit melanin pigmentation and have a whitening effect on the skin (JP 6345797; JP 6321757; JP 6321755; JP5170636; U.S. Pat. No. 5,126,327).
The peptides described in the prior art comprise a high proportion of basic and hydrophobic amino acids and have isoeletric point (pI) values greater than 5.5. These are mainly used for lightening the hyperpigmented areas associated with abnormal skin conditions.
The applicants in their co-pending British patent application 9719195.1, disclose a cosmetic composition for lightening the skin comprising from 0.1 to 10% by weight of a peptide with an isoelectric point of between 2 and 5.5. Isoelectric point (pI) is defined as the pH at which net charge on a molecule is zero. Peptides having large number of acidic amino acids like glutamic acid, aspartic acid etc. have a low pI and those having basic amino acids like lysine, arginine, histidine have a high pI.
The Applicants have found that a composition comprising peptide sequences having a isoelectric point (pI) of between 6 and 11 is capable of darkening the skin/hair.
Accordingly, the present invention relates to a cosmetic skin/hair darkening composition comprising from 0.1% to 10% by weight of a peptide with an isoelectric point (pI) ranging from 6.0 to 11.
The skin/hair darkening effected by the composition of the invention is reversible and without any side effects. The composition according to the invention is active during both day and night.
The peptide is a sequence of amino acids and is of molecular weight ranging from 200 to 20,000 daltons (Da) with a pI ranging from 6.0 to 11.0. The peptide is also optionally linked to a hydrophobic amino acid or a targeting molecule or vehicle.
The amino acid residues forming the peptide sequence can be naturally occurring or synthetic, dextro or levo form, and includes any derivative thereof. The peptide sequence must comprise a proportion of the basic amino acids such that the resulting peptide is basic in nature. The peptide sequence may be straight chain or cyclic.
The molecular weight of the peptide sequence ranges from 200 to 20,000 Da and preferably from 200 to 2000 Da.
The pI of the peptide sequence ranges from 6.0 to 11.0.
The hydrophobic amino acid can be chosen from any one of alanine, isoleucine, leucine, methionine, phenyl alanine, proline, tryptophan or valine and is preferably tryptophan. The targeting molecule is preferable a peptide and most preferably a hexapeptide preferably having the primary sequence disclosed in SEQ ID NO: 1 and is located within 27 amino acid residue from the carboxy terminal of the active peptide. Targeting vehicles such as micelles and/or reverse micelles, may also be used.
According to a preferred aspect of the invention there is provided a cosmetic skin/hair darkening composition comprising from 0.5 to 5.0% by weight of the peptide.
The invention further relates to a cosmetic method of darkening skin/hair comprising topically applying to the skin and/or hair a composition according to the invention. The composition may also comprise a skin tanning agent. This tanning agent may be chosen from any known agent for this purpose such as dihydroxy acetone, theophyllin, copper gluconate, natural actives obtained from Pterocarpus santalinus, and any other known skin tanning agents.
The composition according to the invention may also comprise a cosmetically compatible carrier. It may also comprise preservatives, emulsifiers, thickeners, perfume, colour, skin benefit materials such as moisturisers, emollients and antiageing compounds.
The vehicle which forms part of the cosmetic composition is one or more substances which are compatible with the polyamino acid sequence and which are also cosmetically acceptable in that they will not harm the skin/hair. The vehicles that can be used in the compositions according to the invention can include powder absorbents, binders and carriers, and liquids such as emollients, propellants, solvents, humectants and thickeners. Also simple vehicles such as alcohol, PEG, propylene glycol may also be used.
Examples of moisturisers and humectants include polyols, glycerol, cetyl alcohol, carbopol 934, ethoxylated castor oil, paraffin oils, lanolin and its derivatives. Silicone compounds such as silicone surfactants like DC3225C (Dow Corning) and/or silicone emollients, silicone oil (DC-200 Ex-Dow Corning) may also be used.
The compositions according to the invention may be prepared for topical application to the skin/hair in the form of simple solutions or conventional leave-on or wash-off products such as lotions, creams, ointments, shampoos and/or aerosol products.
All percentages referred to herein and in the appended claims are by weight of the composition unless otherwise indicated.